Something like this perhaps:

use strict;
use warnings;

my %data;

while (<DATA>) {
  next unless /\S/;
  chomp;

  my ($k, $v) = split / = /;
  push @{$data{$k}}, $v;
}

foreach (sort keys %data) {
  print "$_ = ", join(',', @{$data{$_}}), "\n";
}

__DATA__
A = 1
A = 2
A = 3

B = 5
B = 1
B = 7
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